Sound Absorption Properties Of Activated Carbon Fiber Materials

With the rapid development of industrial technology and transportation in the society, noise has become the third source of environmental pollution after air and water, which poses threat to the health of our mankind. Intended to improve the environment in which people live and work, the development of acoustic damping material has become an urgent issue for our society and aroused intensive attention among scholars. The internal structure of activated carbon fiber felts is a unique three-dimensional network, in which there are numerous interconnected micro-pores. Actually, activated carbon fiber felt is a superior sound absorption material and plays a significant role in improving the noise pollution. Therefore, having a systematic study of activated carbon fiber takes great significance.Intended to study the sound absorption properties of of activated carbon fiber felts, viscose-based activated carbon fiber felts acquired by needle punching and activation were prepared, and taken to test the sound absorption properties by means of transfer function method ISO10534-2:1998, within different sound frequencies in the impedance tube. Analysis was made from perspectives of theory and experiment, providing abundant support for the development and design of activated carbon fiber materials. The main results of the study are demonstrated as follows:(1) The influence exerted by technological conditions such as carbonization temperature, carbonization rate, activation temperature and activation time was explored. Among the 13 samples, the activated carbon fiber felts prepared under a carbonization temperature of 500℃, a carbonization rate of 15℃/min, an activation temperature of 850 and an activation time of ℃20 min displayed the best sound absorption properties, coefficients on average reaching as high as 0.62. In a range of frequencies 250-6300 Hz, sound absorption coefficients at medium-low frequencies strikingly climbed with the increasing frequency, while fluctuated at high frequencies. At medium-low frequencies, sound absorption coefficients increased with the rising carbonization temperature and the extended activation time, while subsequently decreased with the rising carbonization rate and carbonization temperature. At the same time, sound absorption coefficients decreased at high frequencies with the rising carbonization temperature and carbonization rate, while increased but later decreased with the rising activation temperature, and increased again with the added time in activation. In summary, activation temperature had the greatest impact on sound absorption properties, then carbonization temperation, activation time and carbonization rate.(2) Influential factors of sound absorption properties are analyzed from perspectives of thickness, bulk density, fiber diameter and so on. The result demonstrated that activated carbon fiber felts with a thickness of 18 mm, a fiber diameter of 8.7?m and a bulk density of 52.8kg.m-3 exhibited exceptional acoustic properties, coefficients arriving at 0.58 on average. It’s found that when thickness was more than 9mm and frequencies were above 630 Hz, coefficients of activated carbon fiber felts all surpassed 0.2. In addition, with a range of frequencies 250-6300 Hz, as frequency increased, sound absorption coefficients of felts with a thickness of 4.5mm went upwards; when thickness amounted to 9mm, sound absorption coefficients showed a sharp increase at medium and low frequencies and a fluctuation at high frequencies. Other technological conditions were kept the same, sound absorption properties were improved first and then impaired with the increase in thickness and bulk density; while with the decrease in fiber diameter, properties were enhanced. It could be observed that the most influential factor was fiber diameter, then thickness and bulk density.(3) Sound absorption properties were studied on four composite structures of activated carbon fiber felts. The results demonstrated that the composite structure displayed different sound absorption properties at different frequencies. Perforated panels played the dominant role in sound absorption by the occurrence of resonance at low and medium frequencies, while porous materials contributed the most at high frequencies. Within an acoustic range of 80-6300 Hz,the composite structure with activated carbon fiber felts inside the perforated panels and rigid walls displayed the best performance in sound absorption, coefficients of felts with a thickness of 5.2mm arriving at 0.52 on average.The composite structure with activated carbon fiber felts attached closely to the front of the perforated panels reached the first resonance frequency at 1650 Hz, which was basically the same with the perforated panel structure, other three composite structures obviously moving to low frequencies with the same scale 400 Hz or so. Composite structures with felts having a thickness of 2.6mm, 5.2mm and 7.8mm, with the increase in the thickness of activated carbon fiber felts, sound absorption coefficients increased, while the thickness was added to 15.6 mm, sound absorption properties of the composite structure had similar traits to that performed by porous materials in an acoustic range of 80-6300 Hz frequencies. With the increase in the distance of air space, sound absorption properties were improving at low frequencies but decreasing at medium frequencies, the first resonance frequency moving towards the low frequencies with a scale of 630 Hz.(4) Predicting model for detecting sound absorption properties of activated carbon fiber was established. Based on models of characteristic impedance and propagation constant proposed by Delany and Bazley, the predicting model of characteristic impedance and propagation constant with acoustic characteristic parameters was established by employing the least square method. Moreover, the model of sound absorbing coefficient was therefore formed and tested, of which the result was compared with the testing result, showing that the result based on predicting model basically coincided with the testing result. The reliability of the established model concerning sound absorbing materials was explored, which provided the theoretical base for developing and designing sound absorbing materials of activated carbon fiber.(5) On the basis of acoustic theories proposed by Zwikker and Kosten, taking into account of the vibrating influence occurring among fibers and modifying the effective fluid density and the effective fluid bulk modulus, theoretical model of propagation constant and characteristic impedance of activated carbon fiber felts was established. Statistics of two acoustic characteristic parameters in theory and trial were compared and contrasted, showing that the theoretical model had its feasibility and can provide reference for developing and designing the activated carbon fiber materials with acoustic characteristics.